'Sensor-to-transmitter trimming' offers precise temperature calibration
Calibrating a good quality temperature transmitter and sensor as a unit, using the "sensor-to-transmitter trimming" technique, results in a precision assembly.
P lant operators rely on the accuracy of their instrumentation for optimal production and safety. In some cases, absolute accuracy is important to control a process at a specific temperature. In others, repeatability to the same operating point, even if it is not an exact temperature, is the goal. Either way, proper selection of a sensor-transmitter assembly is the key to success.
Temperature measurement-as with most things-is limited by its weakest link. Since this will always be the sensor, it is important to select the proper resistance temperature device (RTD) or thermocouple to fit the application.
Sensors, even the most precise models, rarely reflect the accuracy level available from a good transmitter. Absolute accuracy of the measurement relies on how closely the sensor's output matches the data built into the transmitter. The transmitter uses an ideal curve of sensor output versus temperature. Real sensors deviate from their ideal curve.
Better sensor accuracy helps the plant operator keep processes closer to optimum. This results in better, more consistent product quality and reduced production costs.
Basically, temperature calibration consists of comparing the output of a temperature measurement system against a known precision standard. During calibration, the differences are noted and adjustments made accordingly to regulate the instrument to the correct temperature.
The best strategy to achieve the highest accuracy is to calibrate the temperature transmitter and sensor as a unit under very carefully controlled conditions. This technique, termed sensor-to-transmitter trimming, allows the user to adjust for the sensor's output deviation from the ideal curve.
Sensor-to-transmitter trimming requires that the sensor be maintained at a precise temperature while the trimming process is accomplished. This can be done using electrically heated dry block calibrators, with a standard certified probe providing the reference. Higher precision for temperatures under 230 °F (110 °C) is achieved using a portable fluid bath. These baths can be much more precisely controlled, the fluid providing better and faster thermal coupling between the fluid and the sensor being calibrated. For higher temperatures, a dry block or precision oven is used. These procedures are applicable to either RTDs or thermocouples.
The trimming procedure begins by "capturing" one or two temperature points within the operating span configured into the transmitter. These points are typically selected to bracket the normal operating point of the process in which the system will be used. It provides the most precise measurement at the operating point and good accuracy over the desired range.
Finally, a comprehensive report is generated. It certifies that the calibration measurements are traceable to the National Institute of Standards and Technology (NIST, Washington, D.C.) and other applicable criteria established by the manufacturer.
Adding it up
Over a sensor-transmitter assembly's lifetime, the savings due to temperature calibration-greater accuracy, higher yields, and reduced maintenance costs-more than justify the initial cost. Calibrations are performed in an environment that closely resembles the location where the assembly will reside in the field, and may increase the interval between routine calibrations up to five years or more.
In addition, steps are taken to regulate inevitable deviation of the sensor's output from the ideal curve, as needed. For these reasons, when choosing a manufacturer, a company that offers complete sensor/transmitter assemblies calibrated with the sensor-to-transmitter trimming technique provides a solid strategy for accuracy from the very beginning.
J.R. Madden, temperature applications engineer at Moore Industries (North Hills, Calif.).
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